This invention relates to an improved process for preparing an ester derivative of a dihydric phenol by reacting a dihydric phenol with an ester-forming agent to form the corresponding diester and particularly relates to reacting a dihydric phenol with an ester-forming agent having no hydrogens alpha to a carbonyl group of the ester-forming agent. Also, this invention relates to an improved diester process for preparing polyarylate polymers from such a diester derivative of a dihydric phenol. Polyarylates produced by the improved process of this invention have a low degree of coloration, good melt stability and a low level of haze.
Polyarylates are polyesters derived from a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl)propane, also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers which have a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Polyarylates can be prepared by any of the well-known prior art polyester-forming reactions, such as the reaction of the acid chlorides of the aromatic dicarboxylic acids with the dihydric phenols; the reaction of the diaryl esters of the aromatic dicarboxylic acids with the dihydric phenols; or the reaction of the aromatic diacids with diester derivatives of the dihydric phenol. These processes are described in, for example, U.S. Pat. Nos. 3,317,464; 3,948,856; 3,780,148; 3,824,213; and 3,133,898.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diacetate derivative, which is then reacted with one or more aromatic dicarboxylic acid(s) to form a polyarylate. As pointed out in U.S. Pat. No. 4,294,956, light yellow polyarylates can be produced by carrying out this process in a suitable concentration of an appropriate solvent such as diphenyl ether. However, it is desirable to produce polyarylates of even lower color.
The use of high purity, low color diester derivatives of a dihydric phenol monomer is also important in making a low color, stable polyarylate polymer. Conventionally, the purification of such monomers included dissolving the monomers in a suitable solvent, usually with an adsorbent such as activated charcoal, and recrystallizing the product from the solvent. However, such a purification process is complex and uneconomical due to the dissolution and crystallization steps and the large amount of solvent involved.
U.S. Pat. No. 4,075,173 issued Feb. 21, 1978, assigned to Sumitomo Chemical Company, Limited, Osaka, Japan, describes the preparation of copolyesters by reacting an aromatic dicarboxylic acid, a diacetate of Bisphenol-A, and an acetate of p-hydroxybenzoic acid. Various processes for producing polyarylates by the reaction of Bisphenol-A and terephthalic and isophthalic acids are reviewed in this patent. The following process for producing polyarylates, identified as route (1), is described in column 2 of the patent: ##STR1##
This process is the diacetate process as described herein, or the "Acetate Process" as defined in the patent.
Column 2 of the patent states:
"The route (1) is not desirable because the undesirable coloration and deterioration of polymer are particularly remarkable as disclosed in the above-mentioned literature."
Further, column 3 of the patent states:
"On the other hand, the route (1), Acetate process, is economically advantageous because the materials used are cheap and the operation is simple. For example, diacetate of bisphenol-A, a monomer for Acetate process, is synthesized by merely reacting acetic anhydride and bisphenol-A. Consequently, it may be said that, if the fatal drawbacks of Acetate process, color and deterioration, are solved, Acetate process will become the most superior process."
Thus, the skilled workers in the field of polyarylate chemistry realized that processes for producing polyarylates had one or more deficiencies prior to the priority date of U.S. Pat. No. 4,075,173, and that a need existed to develop a viable diacetate process for producing polyarylates.
The process of said U.S. Pat. No. 4,075,173 requires the use of p-hydroxybenzoic acid and produces a particular class of polyarylate copolymers.
Several methods for producing polyarylates by the diacetate process are described in the following U.S. Patent which are commonly assigned to Amoco Corporation, Chicago, Ill. Specifically, these U.S. Patents describe the following:
Commonly assigned U.S. Pat. No. 4,294,956, issued Oct. 13, 1981 in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether" describes a process for preparing polyarylates having a reduced viscosity of from about 0.5 to greater than 1.0 dl/gm, which process comprises reacting at least one diester derivative of a dihydric phenol with at least one aromatic dicarboxylic acid in the presence of a diphenyl ether compound, at a temperature of from about 260.degree. to about 350.degree. C.
Commonly assigned U.S. Pat. No. 4,294,957, issued Oct. 13, 1981, in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates of improved color which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of at least one cycloaliphatic, substituted aromatic or heteroaromatic compound, which compounds contain at least one benzylic and/or tertiary hydrogen atom, at a temperature of from about 260.degree. to about 350.degree. C. Optionally, the process may be carried out in the presence of a magnesium, manganese, or zinc catalyst.
Commonly assigned U.S. Pat. No. 4,296,232, issued Oct. 20, 1981, in the name of L. M. Maresca, et al. and titled "Process For Preparing Polyarylates in the Presence of a Diphenyl Ether Compound and A Catalyst" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of a diphenyl ether compound at a temperature of from about 260.degree. to about 350.degree. C. and in the presence of a magnesium catalyst.
Commonly assigned U.S. Pat. No. 4,321,355, issued Mar. 23, 1982, in the name of L. M. Maresca, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates by a process requiring an additional process step in which prior to reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid the residual acid anhydride is removed from the diester derivative of the dihydric phenol so that its concentration is less than about 1500 parts per million.
Commonly assigned U.S. Pat. No. 4,374,239, issued Feb. 15, 1983, in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates by a process requiring an additional process step in which prior to reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid the residual acid anhydride is removed from the diester derivative of the dihydric phenol so that its concentration is less than about 1500 parts per million.
Commonly assigned U.S. Pat. No. 4,374,239, issued Feb. 15, 1983, in the name of M. H. Berger, et al. and titled "Process For Preparing Polyarylates" describes a process for preparing polyarylates which process comprises reacting a diester derivative of a dihydric phenol with an aromatic dicarboxylic acid in the presence of at least one halogenated and/or etherated substituted aromatic or heteroaromatic compound, at a temperature of from about 260.degree. to about 350.degree. C.
However, the polyarylates produced by the diacetate processes described in U.S. Pat. Nos. 4,296,232; 4,294,956; 4,294,957; 4,321,355 and 4,374,239, supra, still tend to contain colored species to an unacceptable extent for some end uses and tend to be melt unstable if there is too much unreacted anhydride in the dihydric phenol diester composition prior to polymerization. Thus, the polyarylate must be prepared from a purified intermediate dihydric phenol diester, or it is difficult to fabricate. Also, without purification of the diester, the polyarylate may not be acceptable in applications where polyarylates which are low in color are required.
Because use of high color, low purity diester derivatives of a dihyric phenol monomer leads to undesirable properties in polyarylate polymers made therefrom by the diester process, and due to difficulties in conventional methods for purification of the monomers, it is desirable to avoid formation of color body-containing by-products in the formation of the diester. A process which permits formation of diester derivative of a dihydric phenol monomer having, simultaneously, low color and high purity would be useful. A process which permits both without any additional process steps and without an increase in haze or loss in melt stability of polyarylates made therefrom would be very advantageous.
This invention is directed to an improved process for preparing polyarylates which are melt stable, low in color, and low in haze.
The improved diester process of this invention provides a method to prevent formation of undesirable by-products in preparing a diester derivative of a dihydric phenol. For example, in the well-known diacetate process, such extended, conjugated by-products as alpha, beta-unsaturated ketone may be formed by the acylation/Aldol condensation reaction of acylated bisphenol-A-diacetate which introduces a color body into the polyarylate polymer that is difficult to remove.